1. Field of the Invention
The present invention relates to a motor control apparatus configured to control a motor on the basis of a PWM (Pulse Width Modulation) system and, more specifically, to a motor control apparatus configured to detect currents in respective phases by using a single current detector.
2. Description of Related Art
For example, an electric power steering apparatus of a vehicle is provided with an electric motor such as a 3-phase brushless motor in order to provide a steering function with a steering auxiliary power in accordance with a steering torque of a steering handle. Examples of the motor control apparatus configured to control rotations of the motor includes a motor control apparatus on the basis of the PWM system (see JP-A-2010-279141, Japanese Patent No. 4833186, Japanese Patent No. 4884355, Japanese Patent No. 4884356, JP-A-2007-112416, JP-A-10-155278, JP-T-2005-531270, JP-A-2001-95279, and U.S. Pat. No. 6,735,537).
In general, the motor control apparatus on the basis of the PWM system includes an inverter circuit configured to drive a motor on the basis of a PWM signal, a control unit configured to control an action of the inverter circuit, and a current detection circuit configured to detect motor currents. The inverter circuit includes the same number of pairs of upper and lower arms as the number of phases, and each pair of the upper arm and the lower arm is provided with a switching element. The current detection circuit includes a current detection resistance (hereinafter, referred to as “shunt resistance”) configured to detect motor currents in respective phases flowing in the inverter circuit. The control unit generates PWM signals having predetermined duties for the respective switching elements of the inverter circuit on the basis of a deviation between a target value of current to be fed to the motor and a value of the current detected by the shunt resistance, and outputs the generated PWM signals to the inverter circuit. The respective switching elements of the inverter circuit perform ON-OFF operations on the basis of the PWM signals. Accordingly, the current flows from a power source through the inverter circuit to the motor, and hence the motor rotates.
In the case where the shunt resistance configured to detect the motor current is provided on the lower arm for each phase of the inverter circuit, a current in each phase flowing to the motor may be detected as an actual measured value. However, in this case, the same number of shunt resistances as the number of the phases are required, and hence the configuration of a circuit becomes complicated. Therefore, detection of the current in each phase by using a single shunt resistance has been performed in the related art (see JP-A-2010-279141, Japanese Patent No. 4833186, Japanese Patent No. 4884355, Japanese Patent No. 4884356, and JP-A-2007-112416). This system is referred to as “single shunt system”, hereinafter. In the single shunt system, currents in two phases flowing through the shunt resistance are detected, and a current in a remaining phase is obtained by an arithmetic operation on the basis of the detected values (detailed description will be given later).